Vitamin D has become one of the most talked-about nutrients of the decade. From bone health to immunity, cardiovascular disease to cancer prevention, claims about its benefits are everywhere. But separating established science from speculation requires a closer look at the evidence.

Vitamin D is technically a hormone precursor. The active form, calcitriol (1,25-dihydroxyvitamin D), regulates calcium and phosphorus absorption, bone mineralization, and immune function. The body produces vitamin D when UVB radiation from sunlight converts 7-dehydrocholesterol in the skin to cholecalciferol (vitamin D3).

People living at latitudes above 35° (most of Europe, Canada, northern US) don't produce adequate vitamin D from sunlight during winter months. Other risk factors include: dark skin (melanin reduces UVB penetration), obesity (vitamin D is sequestered in fat tissue), elderly age (reduced skin synthesis), limited sun exposure, malabsorption conditions (celiac disease, Crohn's), and certain medications (anticonvulsants, glucocorticoids).

Serum 25-hydroxyvitamin D [25(OH)D] is the standard test. Most guidelines consider <20 ng/mL (50 nmol/L) as deficient and 20–30 ng/mL as insufficient. However, routine screening of asymptomatic individuals is not recommended by most medical societies — testing should be reserved for people at high risk.

For bone health: vitamin D supplementation (combined with calcium) reduces fracture risk in deficient populations, particularly the elderly. For immunity: some evidence supports a modest reduction in respiratory infections with daily (not bolus) vitamin D supplementation. For cancer, cardiovascular disease, and other conditions: the large VITAL trial (25,000 participants) found no significant benefit of vitamin D supplementation for cancer or cardiovascular events in the general population.

Cholecalciferol (D3) is preferred over ergocalciferol (D2) due to better bioavailability and longer duration of action. Standard maintenance doses: 600–1000 IU daily for adults, 1000–2000 IU for those at risk of deficiency, and higher therapeutic doses (e.g., 50,000 IU weekly for 8 weeks) for documented deficiency. The tolerable upper limit is 4,000 IU/day for adults; doses exceeding 10,000 IU/day risk hypercalcemia and kidney stones.

Consult your doctor if you have bone pain, muscle weakness, frequent fractures, or risk factors for deficiency. Don't self-treat with high-dose supplements without testing — vitamin D toxicity, though rare, causes dangerous hypercalcemia (nausea, kidney stones, cardiac arrhythmias). A simple blood test can guide appropriate supplementation.